1. Field of the Invention
The present invention relates to a magnetic recording medium capable of performing high-density magnetic recording, and to a method of producing the same.
2. Description of the Related Art
Recently, a perpendicular magnetic recording medium is drawing attention as a recording medium. Further, an enhancement of plane recording density of a magnetic recording medium such as a hard disk drive (hereinafter, referred to as “HDD”) is expected along with an increase in an information amount, and each recording bit size on the magnetic recording medium is becoming very small (i.e., about several 10 nm).
However, the reduction in a recording bit causes problems in that a magnetization amount per bit decreases and magnetization information disappears due to a magnetization inversion ascribed to “thermal fluctuation”.
In general, the “thermal fluctuation” has a larger effect as a value of Ku·V/kT (herein, Ku: anisotropy constant, V: magnetization minimum unit volume, k: Boltzmann constant, and T: absolute temperature) decreases. It is considered experimentally that when Ku·V/kT becomes less than 100, the inversion of magnetization ascribed to the “thermal fluctuation” occurs.
As a magnetic recording medium for reducing recording sensitivity while keeping durability with respect to “thermal fluctuation”, “Official Journal of The Magnetics Society of Japan” vol. 29, 2005, pages 239-242 by The Magnetics Society of Japan proposes a perpendicular magnetic recording medium in which hard magnetism and soft magnetism are formed in a film thickness direction. FIG. 4 is a view schematically illustrating a magnetic recording film described in “Official Journal of The Magnetics Society of Japan” vol. 29, 2005, pages 239-242 by The Magnetics Society of Japan. The magnetic recording film has a configuration in which a hard magnetic layer 402 made of CoCrPt—SiO2 is formed on a seed layer 401 to be a seed of crystal growth, and a soft magnetic layer 403 made of NiFe—SiO2 is formed after that. This technique is characterized by using a configuration in which a hard layer and a soft layer are laminated so as to keep a magnetic field for recording low while securing thermal stability involved in a reduction of a magnetic region. Here, the magnetic recording film includes a seed layer 401, crystal particles 404, CoCrPt crystals 405, and NiFe crystals 406.
However, with the configuration in which the layers are laminated in the film thickness direction, there is a fear that a film thickness increases. The increase in the film thickness causes a recording magnetic field on a lower portion side (opposite side of a recording head) of a recording film to be diffused easily in a film surface direction, so it is not desirable in a case of enhancing recording density.
On the other hand, the configuration utilizing the hard magnetism and the soft magnetism will become important in the future along with the increase in the recording density.
Thus, the inventors of the present invention have made intensive studies for a new magnetic recording medium having hard magnetism and soft magnetism in an in-plane direction (film surface direction) of a magnetic recording film of a magnetic recording medium, thereby achieving the present invention.